BTW, knowing your operating system would also help here. There is a difference in how much RAM Windows XP Pro (what I'm running) can take vs. Windows 95 or Windows 98 for instance... I'm assuming winXP here...
Let me see if I can help de-mystify a bit, the deal with RAM here. I'm running a system that might be somewhat similar to yours (you do mention DDR, and a Radeon 9700 pro).
My system specs are:
- Athlon XP 1900+ (1.6 GHz AXP)
- 512 MB (256 x 2, Corsair XMS 2400 CAS 2.0, aka 300 MHz DDR, though running at 266 MHz DDR fsb)
- Asus A7N266-E (nForce 420-D chipset)
- Visiontek Radeon 9600
- Windows XP Pro
My system is a bit slow today (wasn't when I got it about 2.5 years ago). I'm looking for an upgrade myself...so in the long run you're going to want an upgrade. However, on my current system I boot up, and the commit charge is already 150-175 MB straight at desktop, before I load games or other apps. I can be over 250 MB just running Mozilla with a couple windows open. With games that are a couple years old, I can use 350-400 MB, and new games, no doubt more. I'm rather expecting to be pushing things when Doom 3 comes out...
Now, what it comes down to is how much your swapping (paging out to disk). The swap file is dealing with virtual memory. In order for the CPU to run something, it needs to be loaded into RAM first...aka when they talk about loading something, they're talking about moving it from where it is stored on the hard drive, into RAM, where the CPU can then execute the program.
However, one can end up using more memory, then they have in their computer, which is where the swapfile, pagefile, whatever one calls it comes into place. This works by first paging the contents of less used pages of memory into virtual memory (or one's swap file), where it can be read back into RAM when the computer needs it. The CPU still needs it in RAM though, to make use of it.
So, if something the CPU needs, is in the pagefile, it first has to be paged back into RAM. I'll give some comparisons on my system, though my hard drives are likely faster then yours.
I have 2 10k rpm Seagate Cheetah drives, sitting on a SCSI bus. Due to SCSI disconnection, both hard drives can be active at the same time, each connecting to the bus only when they have something to send. This effectively allows the transfer rate of each hard drive to be additive, without them interfering with each other.
My newer Cheetah is a generation 3 Cheetah which benched at 35 MB/sec sustained data transfer in Adaptec SCSI Bench 32 when I measured it in Dec 2000. My older, gen 2 Cheetah, same probram benched at about 18 MB/sec sustained. My pagefile is split between these 2 hard drives, with 2/3 on the newer Cheetah, and 1/3 on the older Cheetah. Cumulatively, we're talking potentially 53 MB/sec for my pagefile.
My DDR however, can transer at about 2.1 GB/sec, which is significantly faster then 53 MB/sec. Also locating something in RAM would no doubt be faster then the 5.3 Msec seek time on my Cheetahs.
So, if the memory being used is significantly more then the physical RAM in the computer, a lot of paging will have to take place, and the CPU ends up having to wait on the hard drive to page the stuff back into RAM before it can be used. If one is paging a lot, and they upgrade the CPU, the CPU will spend more time "twiddling it's thumbs" if you will, waiting on the hard drive, till it can do it's thing.
RAM upgrades help, if one can reduce one's reliance upon the swapfile... If one has more RAM then they're using however, a RAM upgrade will not really do much of anything for one, as one isn't really paging anyhow.
If you are in Windows NT/2k/XP, you can right click on the task bar and select Task Manager. Go to the Performance tab (I believe it is, but on the comp I'm on now, it was disabled, so can't check), and look at the commit charge. Check this when you open up some of these games, and alt + tab to the desktop to check this. If this is above 256 MB by a fair degree, a memory upgrade will most likely help you. If you don't exceed this, it won't do so much. I'm assuming your using more then this based on my own experience.
If you're in Windows 95/98, there's something called Performance Monitor, as I remember which allows one to list memory usage, what one's using in the swapfile, and also page faults per second. On the last, I seem to remember it counting more then calls to the page file (the sort of page fault one is interested in), but it's been about 4 years since I used Windows 98, so don't quote me on that one.